Intracavity modulated pulsed laser and methods of using the same

ABSTRACT

An intracavity modulated pulsed laser and methods of using the same. In one preferred form, an intracavity modulated pulsed laser comprises an amplification medium, a pulsed pumping source, a beam modulator, and two mirrors, one totally reflective and one partially reflective for generating at least one laser output burst comprising a plurality of sub-pulses having variably controllable peak powers. In another preferred form, a non-linear crystal is utilized to double the frequency of each laser output burst. In still another preferred form, a fluorescence feedback control circuit is utilized to control the beam modulator.

This application is a continuation-in-part of application Ser. No.07/951,075, filed Sep. 25, 1992, now U.S. Pat. No. 5,390,204, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The field of the present invention is lasers and their methods of use,and more particularly, pulsed lasers and their use in medical, dental,and industrial applications.

Recently, a number of pulsed lasers have been developed for use inmedical, dental, and industrial applications. For example, those skilledin the art will note that pulsed laser systems are commonly provided inthree forms: flashlamp pumped free-running, flashlamp pumpedelectro-optically (E-O) Q-switched, and continuously pumpedacousto-optically (A-O) Q-switched. Further, it will be noted by thoseskilled in the art that the neodymium doped yttrium aluminum garnet(Nd:YAG) laser, a laser which has been adapted for use in numerousmedical and dental applications, is exemplary of pulsed laser systems.

When the Nd:YAG laser is provided as a flashlamp pumped free-runningsystem, a flashlamp pulse having a duration typically between 100 and1000 μs pumps the Nd:YAG rod of the laser, and a laser output pulse ofapproximately the same duration is produced. Flashlamp pumped laserpulse energies are commonly in the 1-1000 mJ range, and for this reasonthe maximum peak laser output power of a flashlamp pumped free-runningsystem is typically 10 kW.

For certain applications, however, those skilled in the art will notethat peak output powers greatly exceeding 10 kW may be desired, andthat, in response to this need, a number of flashlamp pumped E-OQ-switched Nd:YAG lasers have been developed. The flashlamp pumped E-OQ-switched Nd:YAG laser utilizes an electro-optical Q-switch to disruptbeam oscillation within the oscillating cavity of the laser during theentire duration of each flashlamp pulse, and to restore beam oscillationimmediately following each flashlamp pulse. In this fashion,substantially all of the flashlamp pulse energy is stored in the Nd:YAGrod during the duration of each flashlamp pulse, and upon therestoration of beam oscillation within the oscillating cavity a "giantpulse" laser output is produced. More specifically, as beam oscillationis restored, substantially all of the energy stored within the Nd:YAGrod is extracted from the rod in the form of a single giant pulse havinga duration of approximately 10 ns. In this way, laser output pulseshaving peak powers in the range of 100 MW may be readily produced.Unfortunately, laser output pulses having peak powers in the 100 MWrange are not useful in many applications, and in particular, laseroutput pulses having peak powers in 100 MW range cannot be carried byconventional fiber optic delivery systems. While laser output pulseshaving lower peak powers may be obtained from a flashlamp pumped E-OQ-switched Nd:YAG laser by reducing the amount of energy contained ineach flashlamp pump pulse, the difficulty of operating E-O Q-switches athigh repetition rates (i.e. 100 Hz or better) makes it impractical toachieve desirable peak powers while at the same time maintaining anaverage laser output power of between 5 and 50 W.

Finally, in contrast to flashlamp pumped Nd:YAG pulsed lasers,continuously pumped A-O Q-switched Nd:YAG pulsed lasers, such as thelaser disclosed and claimed in U.S. Pat. No. 4,273,535, issued toYamamoto et al., and entitled "Device for Preventing Tooth Decay byLaser Beam Irradiation and Method of Preventing Tooth Decay by Means ofthe Same," typically utilize an arc lamp to continuously pump the Nd:YAGrod, and utilize an A-O Q-switch to periodically trigger energy storageand release by the Nd:YAG rod. The storage time and pulse repetitionrate can be adjusted over a broad range. However, to maximize laseroutput pulse peak powers it is necessary to maximize energy storagewithin the Nd:YAG rod prior to switching. This is accomplished bysetting the rate of triggering in accordance with the lifetime of theexcited state of the Nd:YAG rod. However, those skilled in the art willappreciate that, as the pulse repetition rate is reduced, the efficiencyof a conventional laser is sacrificed, and the average power generatedby the laser decreases. If on the other hand, the pulse repetition rateis increased, the peak powers generated by the conventional laser willdecrease, thus inhibiting any increases in the average power of thelaser. For these reasons, it is impractical using conventional lasersystems to generate laser output pulses having desirable peak powers,while at the same time maintaining an average laser output power in therange of 5-50 W.

Because it is desirable in many applications to generate laser outputpulses having peak powers in the 10 -1000 kW range, while at the sametime maintaining laser efficiency of 1 to 2% or more and maintaining apulse repetition rate in the range of 10-200 Hz, a new laser system isdesired.

SUMMARY OF THE INVENTION

The present invention is directed to an intracavity modulated pulsedlaser and its use. The intracavity modulated pulsed laser is capable ofgenerating output pulses having controllable peak powers in, forexample, the 10-1000 kW range, is capable of maintaining a high averagepower output, and is capable of delivering laser radiation in the formof low frequency bursts. In one preferred form, the intracavitymodulated pulsed laser comprises an amplification medium, a pulsedpumping source, a modulator, and two mirrors, one totally reflective andone partially reflective. The amplification medium is disposed along anoptical axis between the two mirrors. The pulsed pumping source, whichmay comprise, for example, a standard flashlamp, is disposed adjacentthe amplification medium for delivering pulses of pump energy to theamplification medium and exciting the atoms which comprise theamplification medium to elevated quantum-mechanical energy levels. Theintracavity modulator modulates the amplification of a beam oscillatingbetween the mirrors at predetermined intervals during each pulse ofenergy delivered to the amplification medium by the pulsed pumpingsource. In this fashion, rather than generating a conventional singlelaser output pulse in response to each pump pulse delivered by thepumping source, the intracavity modulated pulsed laser generates anoutput burst comprising a plurality of sub-pulses in response to eachpump pulse delivered by the pumping source. Further, when a plurality ofpump pulses are sequentially delivered from the pulsed pumping source tothe amplification medium, the intracavity modulated pulsed laser of thepresent invention will produce an output comprising a plurality ofgrouped sub-pulses or, stated differently, a plurality of multi-pulsedbursts. The sub-pulses of each output burst have substantially increasedand controllable peak powers. To control the peak powers of thesub-pulses comprising each burst, the triggering frequency of modulation(or stated differently, the modulation gating interval) is varied. Aslong as the duration of the period between amplification triggering doesnot approach the lifetime of the excited state of the amplificationmedium, the efficiency of the intracavity modulated pulsed laser is notsacrificed.

It follows that the intracavity modulated pulsed laser embodying apreferred form of the present invention is both efficient and capable ofgenerating an output comprising a plurality of multi-pulsed burstshaving substantially increased and controllable peak powers. Further,because the peak powers of the multi-pulsed bursts are controllable,compatibility between a laser embodying a preferred form of the presentinvention and fiber optic delivery systems may be readily achieved. Morespecifically, by varying the modulation gating frequency (or modulationgating interval) of an intracavity modulated pulsed laser, the maximumpeak powers of the output bursts generated by the laser may becontrolled in accordance with the power tolerance (power density damagethreshold) of various fiber optic delivery systems.

Further, because the peak powers of the sub-pulses comprising eachmulti-pulsed burst are controllable and may be set to a selected,uniform level, a laser in accordance with the present invention providesnumerous advantages in applications requiring second-harmonic beamgeneration or frequency doubling. For example, in a conventional lasersystem the efficiency of conversion from a fundamental beam frequency toa second harmonic frequency will vary proportionally with the peak powerdensity of the fundamental beam. Also, when the average power of aconventional laser system is changed or when the pulse repetition rateof that laser system is changed, the peak powers generated by thatsystem also change. In contrast, where a laser in accordance with thepresent invention is utilized to generate a plurality of multi-pulsedburst outputs, and each burst comprises a plurality of sub-pulses ofselected, uniform peak power, the conversion efficiencies relating tothe respective sub-pulses will vary only minimally and will remainessentially constant. This holds true even over a broad range of averagepowers and burst repetition rates.

Accordingly, it is an object of the present invention to provide animproved laser which is both efficient and capable of delivering anoutput comprising a plurality of output bursts, each comprising aplurality of sub-pulses having substantially increased and controllablepeak powers.

It is a further object of the present invention to provide forutilization in medical, dental, and industrial applications an improvedlaser which is capable of efficiently generating an output comprising aplurality of output bursts having controllable peak powers.

It is still another object of the present invention to provide a methodfor treating, e.g. cutting, ablating, and coagulating, soft tissuesusing multi-pulsed bursts of laser energy.

It is yet another object of the present invention to provide a methodfor treating hard tissues using multi-pulsed bursts of laser energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an intracavity modulated pulsed laser inaccordance with one form of the present invention.

FIG. 2 illustrates the relationship between a conventional flashlamppump pulse and a burst output which may produced therefrom in accordancewith the present invention.

FIG. 3 is a block diagram of an intracavity modulated pulsed laser inaccordance with a preferred from of the invention.

FIG. 4 is an illustration of an acousto-optical Q-switch in accordancewith a preferred form of the present invention.

FIG. 5 is a timing diagram illustrating the relationship betweenflashlamp triggering, RF signal gating, flashlamp discharge, and laseroutput in an intracavity modulated pulsed laser in accordance with apreferred form of the present invention.

FIGS. 6(a) and 6(b) illustrate how output sub-pulse peak powers arecontrolled by varying the modulation frequency of an intracavitymodulated pulsed laser to achieve compatibility with a fiber opticdelivery system having a given power density damage threshold (DT). Inparticular, FIG. 6(a) illustrates a low energy flashlamp pump pulse anda multi-pulsed burst laser output which may be produced therefrom, andFIG. 6(b) illustrates a high energy flashlamp pump pulse and amulti-pulsed burst laser output which may produced therefrom.

FIG. 7 is an illustration of an energy monitor in accordance with apreferred form of the present invention.

FIG. 8 is an illustration of a laser head embodying a preferred form ofthe present invention.

FIG. 9 is a block diagram of a frequency doubled intracavity modulatedpulsed laser in accordance with the present invention.

FIGS. 10(a)-(d) provide an illustration of several exemplary laseroutputs which may be generated using a laser in accordance with one formof the present invention.

FIG. 11 is a flowchart illustrating the sequence of functions performedby a microprocessor in controlling an intracavity modulated pulsed laserin accordance with a preferred form of the present invention.

FIG. 12 is a block diagram of an intracavity modulated pulsed laseremploying a fluorescence feedback circuit in accordance with the presentinvention.

FIG. 13 illustrates how sub-pulse peak powers are controlled using thefluorescence feedback circuit illustrated in FIG. 12.

FIG. 14 is an illustration of a control panel of an intracavitymodulated pulsed laser in accordance with a preferred form of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to the drawings, FIG. 1 is a block diagram of an intracavitymodulated pulsed laser 1 in accordance with the present invention. Asshown, the laser 1 comprises an amplification medium 2, a pulsed pumpingsource 4, a modulator 6, a totally reflective mirror 8, and a partiallyreflective mirror 10. The amplification medium 2 is disposed along anoptical axis 12 between the two mirrors 8 and 10, and the pulsed pumpingsource 4, which may comprise, for example, a conventional flashlamp, isdisposed adjacent the amplification medium 2 for delivering pulses ofpump energy to the amplification medium 2 and exciting the atoms whichcomprise the amplification medium 2 to elevated quantum-mechanicalenergy levels. As the atoms comprising the amplification medium returnto their initial or lower quantum-mechanical energy levels, photons ofpredetermined wavelengths will be spontaneously emitted by those atoms,and a number of the spontaneously emitted photons will trigger furtherphoton emissions ("stimulated emissions"). A number of the stimulatedemissions will then form a beam 14 which oscillates between the mirrors8 and 10. Finally, the oscillation of the beam 14 between the mirrors 8and 10 will trigger further stimulated emissions which will cause thebeam 14 to be amplified.

The modulator 6, which may comprise for example an acousto-opticalQ-switch or a saturable absorber dye, is also disposed along the opticalaxis 12 between the mirrors 8 and 10. The modulator 6 functions toreduce stimulated emissions within the amplification medium (i.e. tominimize lasing or amplification of the beam 14 as it travels within theamplification medium) by disrupting the oscillation of the beam 14between the mirrors 8 and 10. This process is commonly referred to asswitching the quality or "Q" of a laser oscillator. The modulator 6 alsofunctions to increase the Q of the laser 1 at predetermined intervalswithin each pulse of energy generated by the pulsed pumping source 4,thus causing oscillation and amplification of the beam 14 at each suchinterval. By repeatedly modulating the oscillating beam 14 of the laser1 during each energy pulse generated by the pulsed pumping source 4,substantial population inversions are repeatedly built up within theamplification medium 2, and highly accelerated depletions of thoseinversions are triggered upon increasing the Q of the laser 1. In thismanner, a laser output 15 comprising one or more multi-pulsed burstshaving peak powers in the range of 10-1000 kilowatts may be readilygenerated. Further, as long as the interval between the sub-pulsescomprising each multi-pulsed burst does not approach the lifetime of theexcited state of the amplification medium 2 of the laser 1, spontaneousemission energy losses will be minimized, and the efficiency of thelaser 1 will not be sacrificed. The relationship between a typicalflashlamp pump pulse 11 and an exemplary multi-pulsed burst laser output15 which may be produced therefrom is illustrated in FIG. 2.

Turning now to FIG. 3, in a preferred form the amplification medium 2 ofthe intracavity modulated pulsed laser 1 comprises a standard, 1%neodymium doped, yttrium aluminum garnet rod (Nd:YAG rod) measuring 5 mmin diameter and 80 mm in length. The dimensions of the Nd:YAG rod willvary, however, depending upon the average power output and dependingupon the particular beam diameter which is desired from the laser 1. Itwill also be noted that Nd:YAG rods of the type described may bepurchased from any one of a number of laser component distributorsincluding Lightning Optical of Tarpon Springs, Fla. Further, it ispresently preferred to apply a single layer anti-reflective (AR) coatingto the ends of the Nd:YAG rod. The AR coating may be centered at any oneof a number of wavelengths including, for example, 1.064 μm, 1.320 μm,and 1.440 μm, depending upon the anticipated field of use of thelaser 1. Although testing is ongoing, the 1.064 μm and the 1.320 μmwavelengths have demonstrated utility in hard and soft tissueapplications. The utilization of amplification mediums other thanneodymium doped yttrium aluminum garnet crystal may require theutilization of AR coatings centered at other wavelengths. For example,it might be desirable to coat the ends of an erbium doped yttriumaluminum garnet rod with an AR coating centered at 2.910 μm.

With respect to the pulsed pumping source 4, a 450 Torr Xenon flashlampis presently preferred. Flash-lamps of this type are available from ILC,Inc., of Sunnyvale, Calif, and the ILC Model L7652 is preferred. Theenergy delivered to the Nd:YAG rod 2 by the flashlamp 4 is controlled bya microprocessor 20 which is coupled to the flashlamp power source 22.The microprocessor 20 may comprise, for example, an Intel Model 8031with EEPROM, and as illustrated, the microprocessor 20 delivers twosignals S1 and S2 to the flashlamp power source 22. The first signal S1controls the amount of energy which is delivered from the power source22 to the flashlamp 4 and, in turn, the amount of energy which isdelivered from the flashlamp 4 to the Nd:YAG rod 2. The second signal S2triggers the discharge of the flashlamp power source 22 and, thus,controls the timing or repetition rate of the pulses generated by theflashlamp 4.

Referring now also to FIG. 4, in a preferred form the beam modulator 6of the intracavity modulated pulsed laser 1 comprises an acousto-opticalQ-switch 24. In particular, it is presently preferred to use anacousto-optical Q-switch comprising a lithium niobate (LiNbO₃)transducer 26 bonded to an SF10 glass crystal 28. Acousto-opticalQ-switches of this type may be purchased, for example, from Neos, Inc.,of Melbourne, Fla. However, it should be noted that numerous otheracousto-optical Q-switches, including standard quartz switches, may beutilized. Further, while it is generally accepted that the acousticaperture of the crystal 28 should match the diameter of the oscillatingbeam 14, it has been found that the acousto-optical Q-switch 24 operatesmore efficiently if a slightly smaller acoustic aperture is used. Anacoustic aperture measuring 4 mm by 15 mm is presently preferred.

Turning now also to the timing diagram depicted in FIG. 5, the modulator6 is preferably driven at a frequency of 27.12 Mhz by a RF driver 26. RFdrivers capable of sustaining a 27.12 Mhz drive frequency at 10 wattspeak power are available, for example, from Neos, Inc., of Melbourne,Fla. The initial activation and gating of the RF driver 26 is preferablycontrolled by the microprocessor 20. For example, as shown in FIG. 5, ineach instance that the microprocessor 20 delivers a trigger signal S2(shown as waveform A) to the flashlamp 4 causing the flashlamp 4 todischarge a pump pulse 11, the microprocessor simultaneously delivers anRF driver signal S3 (shown as waveform B) to the RF driver 26 and the RFdriver signal S3 switches (or gates) the RF driver ON and OFF atpredetermined intervals during the period of the flashlamp pump pulse 11(shown as waveform C). In one preferred form, the period of theflashlamp discharge pulse 11 is approximately 100 μs, and the gatingfrequency of the RF driver signal S3 is set between 10 kHz and 300 kHz.Thus, a laser output 15 (shown as waveform D) comprising a multi-pulsedburst of between 2 and 50 sub-pulses is produced from each flashlamppump pulse 11. Importantly, by varying the gating frequency of the RFdriver signal S3, it is possible to control the peak powers of themulti-pulsed bursts which comprise the laser output 15. For this reason,the gating frequency of the RF driver signal S3 is controlled by themicroprocessor 20. More specifically, the microprocessor 20 determinesan optimum gating frequency (or optimum gating intervals) based onpreset peak power and gating information which is stored in its memory.The optimum gating frequency of the preferred embodiment is thefrequency at which the peak power of the largest sub-pulse in amulti-pulsed burst attains a prescribed threshold level L1 just belowthe power density damage threshold level DT of a fiber optic deliverysystem 40 used in conjunction with the intracavity modulated pulsedlaser 1. However, it should be understood that the gating frequency ofsignal S3 need not remain constant over the duration of a flashlamp pumppulse. Thus, in accordance with the present invention optimum gatingintervals may be established such that the peak powers of the sub-pulsescomprising each multi-pulsed burst do not exceed the damage threshold ofan associated laser delivery system.

In another preferred form, the microprocessor 20 will still be used totrigger the flashlamp pulse and turn on the RF driver. However, in thisinstance the gating of the RF driver (i.e. the modulator) is controlledthrough the microprocessor 20 by a real-time feedback circuit (describedbelow with reference to FIGS. 12 and 13) which monitors the gain of thelaser cavity.

An exemplary relationship between the amount of pump pulse energydelivered by the flashlamp 4 to the Nd:YAG rod 2 and the optimum gatingfrequency of the RF driver signal S3 for use with a fiber optic deliverysystem having a given power density damage threshold DT is illustratedin FIGS. 6(a) and 6(b). As shown in FIG. 6a, if a small pump pulse, forexample, a 50 mJ pulse pump, is delivered from the flashlamp 4 to theNd:YAG rod 2, and the fiber optic damage threshold DT comprises 100 kW,the frequency of the RF gating signal S3 will be set by themicroprocessor 20 to approximately 50 kHz. If on the other hand, asshown in FIG. 6(b), a large pump pulse, for example, a 320 mJ pumppulse, is delivered from the flashlamp 4 to the Nd:YAG rod 2 in the samesystem, the frequency of the RF gating signal S3 will be set by themicroprocessor 20 to approximately 320 kHz. Thus, it will be appreciatedthat to maintain a given peak power threshold level L1 the optimumgating frequency of the RF driver will be increased as the amount ofenergy contained in each pump pulse P1 is increased, and the optimumgating frequency of the RF driver will be decreased as the amount ofenergy contained in each pump pulse P1 is decreased. It follows that byreprogramming the microprocessor 20 compatibility between theintracavity modulated pulsed laser 1 and virtually any fiber opticdelivery system 40 may be readily achieved.

Turning again to FIG. 3, in a preferred form a beam energy monitor 27 isutilized to measure the energy contained in each output burst 15generated by the intracavity modulated pulsed laser 1 and to providefeedback to the microprocessor 20. This enables the microprocessor 20,in response to variations which may occur over time between anticipatedlaser output energies and actual laser output energies, to automaticallyadjust the amount of energy which is delivered from the flashlamp powersource 22 to the flashlamp 4 at a given pump pulse energy setting. Apresently preferred energy monitor is illustrated in FIGS. 7 and 8 andcomprises a shallow angle beam splitter 29, which picks off roughly 4%of the laser output beam 15 forming a sampled beam 31, and directs thesampled beam 31 toward an alumina diffuser 33. The alumina diffuser 33diffuses the sampled beam 31 forming a diffused beam 35, and a portionof the diffused beam 35 passes through a hole 37 located in the baseplate 39 of the laser head 41 and strikes a germanium diode 43. Thegermanium diode 43, upon being struck by the diffused beam 35, generatesa current which is proportional to the intensity of the diffused beam35, and the generated current is integrated over time and digitized byan electronic integrator (not shown). It will be noted by those skilledin the art that in this fashion the laser output beam 15 is integratedspatially by the diffuser 33 and temporally by the electronicintegrator. Thus, the resulting signal S4, which is delivered to themicroprocessor 20, is proportional to the laser output burst energy andis independent of pulsewidth and spatial variation.

As shown in FIGS. 3 and 14, in a preferred form a control panel 30 isprovided for conveying information indicative of a desired laser outputburst energy and a desired burst repetition rate to the microprocessor20. The control panel 30 is under the control of a second microprocessor(not shown), which is coupled to the control system microprocessor 20 byconventional means.

As for the mirrors 8 and 10, it is presently preferred that the totallyreflective mirror 8 comprise a convex mirror 10 having a radius ofcurvature of approximately 50 cm, and that the partially reflectivemirror comprise a concave mirror having a radius of curvature ofapproximately 60 cm. The utilization of a concave/convex mirrorarrangement minimizes variations in the diameter of the oscillating beam14, as the amount of energy delivered to the Nd:YAG rod 2 by theflashlamp 4 increases. In doing so, the concave/convex mirrorarrangement minimizes potential damage to the input face (not shown) ofthe optical fiber 34. In addition, depending upon the anticipated fieldof use of the intracavity modulated pulsed laser 1, it is presentlypreferred to coat the mirrors with a multi-layer dielectric V coating.As set forth above in the discussion concerning the AR coatingsdeposited over the ends of the Nd:YAG rod 2, the multi-layer dielectricV coating may be centered at any of a number of wavelengths, including1.064 μm, 1.320 μm, and 1.440 μm. Further, for soft tissue applications,coatings centered at 1.064 μm and 1.320 μm have demonstrated utility,whereas for certain hard tissue applications, such as the vaporizationof dental enamel, a coating centered at 1.320 μm is presently preferred.It should be noted, however, that the precise coating used will dependupon the particular amplification medium utilized by the system, andthat for this reason the identification of particular preferredwavelengths is not intended to limit the scope of the invention in anyway. With respect to the reflectivity of the mirrors 8 and 10, it ispresently preferred to coat the totally reflective mirror 8 with amulti-layer dielectric V coating having a reflectivity of 99.5% orbetter. The partially reflective mirror should be coated with amulti-layer dielectric V coating having a reflectivity of approximately60% if a laser output beam 15 having a wavelength of 1.064 μm isdesired, and a reflectivity of 90% if a laser output beam 15 having awavelength of 1.320 μm is desired.

Because it is anticipated that various embodiments of the intracavitymodulated pulsed laser 1 will be employed in medical and dentalapplications, and because neither beams having a wavelength of 1.064 μmnor beams having a wavelength of 1.320 μm are visible to the human eye,in a preferred form the laser output beam 15 is combined with a visibleaiming beam 36 generated by a helium neon (HeNe) laser 38 or a diodelaser (not shown) using a conventional beam combining mirror 42. Thus,to aim the output beam 15 of the laser 1 at a specified tissue area apractitioner using the laser 1 may merely direct the visible aiming beam36 at that tissue area.

As shown in FIG. 8, which provides an illustration of a laser head 41 inaccordance with a preferred form of the present invention, it ispresently preferred to house the flashlamp 4 and the Nd:YAG rod 2 withinan optical pump chamber 45 having a pair of optical openings 47 disposedalong the optical axis 12 (shown in FIGS. 1 and 3). The optical pumpchamber 45, in addition to providing a housing for the Nd:YAG rod 2 andthe flashlamp 4, provides a conventional coolant flow system 49 whichreceives coolant (e.g. water) from a cooling system (not shown) anddelivers that coolant to the Nd:YAG rod 2 and the flashlamp 4. Opticalpump chambers of the type disclosed herein may be purchased, forexample, from Big Sky Laser in Bozeman, Mont.

Now, turning to a discussion of a number of exemplary applications foran intracavity modulated pulsed laser in accordance with a preferredform of the invention, it is presently anticipated that intracavitymodulated pulsed lasers, such as those described above, will meet anumber of needs in the fields of medicine and dentistry, as well as innumerous other fields and applications. Moreover, it is submitted thatintracavity modulated pulsed lasers, such as those described above, willbe found useful in any field or application which requires an efficientlaser capable of delivering output pulses having increased orcontrollable peak powers.

One field, wherein it is anticipated that a laser in accordance with thepresent invention will prove to be quite useful, is the field of secondharmonic beam generation or frequency doubling. More specifically, inlaser systems second harmonic beam generation or frequency doubling maybe achieved through the use of non-linear birefringent crystals.Further, the efficiency of conversion from a fundamental frequency to asecond harmonic frequency is proportional to the power density of thefundamental beam and, therefore, the power of the second harmonic beamis proportional to the square of the power of the fundamental beam.Thus, to provide a practical laser device capable of generating afrequency doubled beam, it is necessary to use a laser source capable ofgenerating a beam having a high power density. In conventional lasersystems, an E-O Q-switched laser capable of producing high power laseroutput pulses provides the laser power source, and the output pulsesgenerated by the E-O Q-switched laser are provided to and passed througha non-linear crystal. However, as is the case with virtually all E-OQ-switched laser systems, it is quite difficult to control the powerdensity of the output pulses generated by such a system and, therefore,difficult to utilize such a system with conventional fiber opticdelivery systems. In contrast, where an intracavity modulated pulsedlaser in accordance with the present invention is utilized to generate aplurality of multi-pulsed output bursts to be provided to a non-linearcrystal and to produce a plurality of frequency doubled output bursts,the peak powers of the sub-pulses comprising the output bursts may becontrolled and set to a level not exceeding the threshold of anassociated fiber optic delivery system.

An exemplary embodiment of an intracavity modulated pulsed laser 70including a non-linear crystal 72 for frequency doubling is illustratedin FIG. 9. It will be noted that the intracavity modulated pulsed laser70 illustrated in FIG. 9 and the intracavity modulated pulsed laser 1illustrated in FIG. 3 comprise virtually the same components andfunction in virtually the same manner; the only difference between thetwo systems being that the intracavity modulated pulsed laser 70illustrated in FIG. 9 includes a non-linear crystal 72. In a preferredform, the non-linear crystal 72 may comprise apotassium-titanyl-phosphate (KTP) crystal manufactured by LittonIndustries (Airtron Division) of Morris Plains, N.J. The non-linearcrystal 72 functions to convert a multi-pulsed output burst generated bythe laser 1 from a wavelength of 1.064 μm to a wavelength of 532 nm(i.e. to a multi-pulsed burst having a frequency in the green spectrum).

Finally, as pointed out above, the use of an intracavity modulatedpulsed laser 1 as a source for a frequency doubled system provides atleast one substantial advantage in addition to allowing the frequencydoubled system to be readily used with conventional optical fiberdelivery systems--an intracavity modulated pulsed laser 1 provides ameans for generating a plurality of multi-pulsed output bursts, eachcomprising a plurality of sub-pulses having selected, uniform peakpowers. Moreover, because the peak powers of the sub-pulses comprising amulti-pulsed output burst may be controlled in accordance with thepresent invention by varying the gating frequency or gate triggering ofthe RF driver 26, it is possible to control the gating frequency or gatetriggering of the RF driver 26 in such a fashion that the sub-pulsescomprising each multi-pulsed burst output have not only controlled, butalso, uniform peak powers. As an example, the relationships between atypical flashlamp pump pulse 80 and three representative laser outputswhich may be produced there from are illustrated in FIGS. 10(a)-(d). Ifthe pump pulse illustrated in FIG. 10(a) is delivered from the flashlamp4 to the Nd:YAG rod 2, and the laser output produced thereby is notmodulated, a laser output pulse similar to that illustrated in FIG.10(b) will result. If, on the other hand, the laser output pulse ismodulated by triggering the gating of the RF driver 26 at a constantfrequency (i.e. by setting the gating frequency of the RF driver to aconstant frequency), a laser output similar to the multi-pulsed burstillustrated in FIG. 10(c) will result. Finally, if the laser output ismodulated by gating the RF driver 26 at a varying frequency during theduration of the pump pulse (i.e. at a decreased frequency while the pumppulse has a relatively low power and an increased frequency while thepump pulse has a relatively high power), a laser output similar to themulti-pulsed burst illustrated in FIG. 10(d) will result. By varying thegating frequency of the RF driver 26 over the duration of the pumppulse, it is possible to produce a multi-pulsed burst output comprisinga plurality of sub-pulses having not only controllable, but also,uniform peak powers. Where a frequency doubled laser output is to beutilized, the generation of sub-pulses having uniform peak powers ishighly desirable, and the use of an intracavity modulated pulsed lasersystem in accordance with the present invention to be quite useful as alaser source for a frequency doubled laser system.

With respect to the medical and dental fields, the utility of the Nd:YAGlaser has already been demonstrated. Specifically, it has been foundthat for numerous applications the Nd:YAG laser can be a highlyeffective tool. In part, this is because the 1.064 μm beam which isproduced by the Nd:YAG laser may be carried with minimal energy lossesby standard optical fibers. The difficulty which has been encountered byprior art systems, however, is that those systems have been unable tomaintain a high average power output, while at the same time deliveringlaser output pulses which have increased peak powers and may betransmitted using conventional fiber optic delivery systems. For thisreason treatment, including tissue vaporization and ablation, using thesystems of the prior art requires substantial time. In contrast, becausean intracavity modulated pulsed laser maintains a high average poweroutput while at the same time providing laser output pulses havingincreased peak powers, tissue vaporization and ablation, for example,using an intracavity modulated pulsed laser proceeds at a much morerapid rate. In addition, because the intracavity modulated pulsed laserdelivers multi-pulsed energy bursts to an area of tissue to be treated,thermal damage to adjacent tissues is minimized.

As set forth above, when using an intracavity modulated pulsed laser fortreating hard and soft tissue disorders, it is preferable to couple theintracavity modulated pulsed laser to a fiber optic delivery system 40.Fiber optic delivery systems 40 generally comprise a optical fiber 34which is coupled to a standard SMA 905 connector 44 at one end, andwhich has a conventional laser hand piece 46 coupled to the other end.The connector 44 is adapted to engage the fiber coupler 32 (shown inFIGS. 3 and 8) such that both the aiming beam 36 and the laser outputbeam 15 may be carried by the fiber 34 to the hand piece 46. Hand pieces46 are generally of one of two types, contact and non-contact. If anon-contact hand piece is utilized, the beams 15 and 36 are generallydeflected at a prescribed angle and focused at a prescribed distancefrom the hand piece 46 by conventional means. To aim the laser outputbeam 15 using a non-contact handpiece a practitioner must only directthe visible aiming beam 36 toward a prescribed tissue area. Once this isaccomplished, the laser 1 may be "fired", activated by depressing the"fire" switch 48. If, on the other hand, a contact hand piece isutilized, the beams 15 and 36 will be delivered to a contact tip (notshown), and when the contact tip is placed in contact with a prescribedtissue area, the beams 15 and 36 will be delivered directly to thattissue area at the point of contact. Thus, when using a hand piece ofthe contact variety, the practitioner may commence firing the lasereither prior to or after bring the contact tip into contact with adesired tissue area.

In each instance of firing, in a preferred form the microprocessor 20will proceed through the steps set forth in FIG. 11. Specifically, themicroprocessor 20 will perform an initialization step (100) by readingthe mode setting, the output energy setting, and the flashlamp pulserepetition rate setting, all of which are manually entered by theoperator. The mode setting determines whether a conventional pulsedlaser output or a multi-pulsed burst output will be delivered by theintracavity modulated pulsed laser. The output energy setting determinesthe amount of energy which will be delivered by the laser 1 within eachconventional output pulse or each multi-pulsed output burst (dependingupon the mode setting). The flashlamp pulse repetition rate settingdetermines the number of flashlamp pulses per second which will bedelivered by the flashlamp 4 to the Nd:YAG rod 2 during continuouslyrepeated firings of the laser.

After initialization (100), the microprocessor 20 will perform flashlampenergy calculation step (110). Specifically, the microprocessorcalculates and sets the amount of energy to be delivered from theflashlamp power source 22 to the flashlamp 4. This calculation isperformed based on information stored in a look-up table, which isstored in the memory of the microprocessor 20. The information in thelook-up table is based on a correction factor which is determined basedon the previous output energy setting and the actual amount of energydelivered by the laser 1 within each conventional output pulse or eachmulti-pulsed burst as the case may be. The actual amount of energydelivered by the laser 1 is measured by the energy monitor 27. In thisfashion, the microprocessor 20 compensates for changes in laserefficiency which will occur over time.

Next, the microprocessor 20 will set the firing mode of the laser (120)to a conventional or a multi-pulsed burst setting in accordance withpreviously read mode information. When conventional mode has beenselected, the microprocessor 20 will wait to receive a fire signal fromthe fire switch 48 (130), and after receiving the fire signal from thefire switch 48, the microprocessor will trigger a flashlamp pulse (140).If the multi-pulsed burst output mode has been selected, themicroprocessor 20 will set the optimum gating frequency, or gatingsequence, of the RF driver signal S3 (150), and wait for a fire signalfrom the fire switch 48 (160). Upon receiving a fire signal from thefire switch 48, the microprocessor 20 will, as shown in FIG. 5,simultaneously trigger a flashlamp pulse and commence the oscillationand gating of the modulator 6 (170). After the flashlamp pulse haspassed, the microprocessor 20 will turn off the oscillation and gatingof the modulator 6 (180). In either mode, the laser output energy willbe measured by the energy monitor 27, and a signal 54 indicative of thelaser output energy will be provided by the energy monitor 27 to themicroprocessor 20 (190). The microprocessor 20 will then wait for onepulse period (step 200) and repeat the sequence.

Turning now also to FIGS. 12 and 13, in an alternative mode, the gatingof the RF driver signal S3 may be controlled using a fluorescencefeedback circuit. An intracavity modulated pulsed laser 80 incorporatinga fluorescence feedback circuit 82, 84 and 86 is illustrated in FIG. 12.In a preferred form, the fluorescence feedback circuit may comprise, forexample, a germanium photoelectric cell 82 manufactured by GermaniumPower Devices (part number GM5) of Andover, Mass.; a 1.06 μm narrow passfilter 84 manufactured by Corion (part number SD 4-1064-F) of Hollister,Mass.; and a comparator 86. The photo-electric cell 82 is disposedwithin the pump chamber of the laser 80 adjacent (or, stateddifferently, in the vicinity of) the Nd:YAG rod 2. The narrow passfilter 84 is mounted to the photo-electric cell in a conventionalfashion, and the photo-electric cell 82 is electrically coupled to oneinput of the comparator 86 which, in turn, is electrically coupled tothe microprocessor 20.

When a fluorescence feedback control system is utilized in anintracavity modulated pulsed laser in accordance with the presentinvention, the microprocessor 20 triggers the flashlamp pulse dischargesand commences the oscillation of the modulator 6 (i.e. enable thedelivery of the RF driver signal S3 to the RF driver 26). However, thegating of the modulator 6 and, more particularly, the gating of the RFdriver signal S3 is controlled in real-time, through the microprocessor20, by the fluorescence feedback circuit 82, 84, and 86 which monitorsthe gain of the laser cavity and delivers a signal S5 to themicroprocessor 20 when the gain of the laser cavity reaches a prescribedlevel (i.e. when the fluorescence of the Nd:YAG rod 2 reaches aprescribed level). Upon receiving the signal S5 from the comparator 86(indicating that the gain of the laser cavity has reached apredetermined level) the microprocessor 20 will gate the signal S3 beingprovided to the RF driver 26 for a period on the order of 2 μs, and anoutput pulse (or sub-pulse, as the case may be) will be emitted by thelaser 80. This process will continue over the duration of each pumppulse provided by the flashlamp 4, enabling the laser 80, in response toeach pump pulse provided by the flashlamp 4, to produce a multi-pulsedoutput burst comprising a plurality of sub-pulses having predetermined,uniform peak powers. The microprocessor 20 may be programmed to adjustthe reference voltage V_(ref) of the comparator 86 in a conventionalfashion and, the microprocessor 20 may be used to select the gain valueat which the signal S5 will be sent to the microprocessor 20 by thecomparator 86.

The function of the feedback circuit 82, 84 and 86 may be explained asfollows. The amplitude and width of a Q-switched pulse for a fixed lasercavity geometry depends on the gain in the cavity and, for a fixedcavity geometry, the gain is dependent on the number of excited Nd ionsin the Nd:YAG rod 2. The rate of spontaneous emission and fluorescenceof the Nd:YAG rod 2 are also proportional to the number of excited ions.Thus, by monitoring the fluorescence of the Nd:YAG rod 2 and, in doingso, the rate of spontaneous emission from the upper laser level, thegain in the cavity can be monitored. In the laser 80 illustrated in FIG.12, the photo-electric cell 82 generates a signal S7, i.e. a voltageS_(S7), which is proportional to the detected fluorescence of the ND:YAGrod 2. Signal S7 is provided to one input of the comparator 86 and, whencompared to a reference voltage V_(ref) (determined by signal S6, whichis provided to the comparator 86 through D/A converter 88), determinesthe output state of the comparator 86. For example, when voltage V_(S7)exceeds the reference voltage V_(ref), the output (Signal S5) will behigh; and when voltage V_(S7) does not exceed the reference voltageV_(ref), the output (Signal S5) will be low. Thus, when the rate ofspontaneous emission reaches a critical value, and signal S5 goes high,the modulator 6 will be gated, i.e., the Q-switch will be opened forapproximately 2 μs, and a pulse (or, more appropriately, a sub-pulse)will be emitted by the laser 80.

When a fluorescence feedback circuit is used in accordance with thepresent invention, the multi-pulsed bursts emitted by the laser 80 willbe highly uniform in amplitude and duration. This is so because the gainof the laser cavity will be essentially equal for each sub-pulsecomprising the emitted multi-pulsed bursts. However, by the referencevoltage V_(ref) over the duration of a flashlamp pulse, themicroprocessor 20 may set the peak power of each sub-pulse comprising agiven output burst to virtually any desired level within the operatingparameters of the system. These unique capabilities are particularlyadvantageous because, using these capabilities, automatic compensationmay be provided for many changes, which may occur over time, in theoperating characteristics of the laser system 80. For example,compensation for changes resulting from aging of the flashlamp 4 may beautomatically provided.

The fluorescence feedback circuit 82, 84 and 86 need not necessarilyprovide a gating signal S5 to the microprocessor 20. The gating signalS5 might be provided directly to the RF driver 26. In such anembodiment, the microprocessor would enable the RF driver 26 (i.e.commence the oscillation of the A-O Q-switch 24), but the gating of theoscillation of the A-O Q-switch would be controlled by the provision ofthe gating signal S5 directly to the RF driver 26.

Turning now also to FIG. 14, in a preferred form the desired laseroutput burst energy setting and flashlamp pulse repetition rate settingare conveyed to the microprocessor 20 by the control panel 30. As shownin FIG. 14, the burst energy setting may be adjusted by manipulating apair of buttons 50 labeled (+) and (-) which are located just below thepulse energy display 52 on the control panel 30. The pulse repetitionrate may be adjusted by manipulating a pair of buttons 54 labeled (+)and (-) which are located just below the pulse repetition rate display56 on the control panel 30. The multi-pulsed burst mode may be selectedby depressing a button 58 labeled accordingly. In the presentlypreferred form, the burst energy setting may be set between 30 mJ and320 mJ, and the flashlamp pulse repetition rate may be set between 10 Hzand 100 Hz.

Assuming that a pump pulse having a duration of approximately 100 μs isdelivered by the flashlamp 4 to the Nd:YAG rod 2, suggested burst orpulse energy settings, pulse repetition rate settings, mode settings,and RF gating frequencies (if applicable) for various soft tissueapplications (including incision, excision, ablation, and coagulation)are set forth in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                               AVG  RF                                                            ENERGY                                                                              RATE POWER                                                                              GATING                                        APPLICATION                                                                            MODE   (mJ)  (Hz) (W)  FREQUENCY                                     __________________________________________________________________________    Incision Multi-pulsed                                                                         30-200                                                                              20-50                                                                              3-10 30-200 kHz                                             Conventional                                                                         30-200                                                                              20-50                                                                              3-10 Off                                           Excision Multi-pulsed                                                                         30-200                                                                              20-50                                                                              3-10 30-200 kHz                                             Conventional                                                                         30-200                                                                              20-50                                                                              3-10 Off                                           Ablation Multi-pulsed                                                                         30-100                                                                               50-100                                                                            3-10 30-100 kHz                                             Conventional                                                                         30-100                                                                               50-100                                                                            3-10 Off                                           Coagulation                                                                            Conventional                                                                         30-100                                                                               30-100                                                                            3-10 Off                                           __________________________________________________________________________

Assuming that a pump pulse having a duration of approximately 100 μs isdelivered by the flashlamp 4 to the Nd:YAG rod 2, suggested burst orpulse energy settings, flashlamp pulse repetition rate settings, pulsemode settings, and RF gating frequencies for hard tissue ablation areset forth in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                   RF                                                                       AVG  GATING                                                         ENERGY                                                                              RATE                                                                              POWER                                                                              FREQUENCY                                      APPLICATION                                                                            MODE   (mJ)  (Hz)                                                                              (W)  (kHz)                                          __________________________________________________________________________    Ablation Multi-pulsed                                                                         100-320                                                                             30-100                                                                            3-10 100-320                                        Ablation Conventional                                                                         200-320                                                                             30-100                                                                            6-10 Off                                            __________________________________________________________________________

Finally, with respect to certain hard tissue applications and, inparticular, with respect to the ablation of hard tissue, i.e., thedrilling of holes in dental enamel, it appears, based on experimentationto date, that it may be preferable to utilize a beam having a wavelengthof 1.320 μm. However, because not all fibers are capable of efficientlycarrying a beam having a wavelength of 1.320 μm, it is preferable toutilize a low OH⁻ fiber to carry the laser output beam 15 to the handpiece 46. Fibers of this type are available, for example, from PolyMicro Technologies, Inc., of Phoenix, Ariz., and model number FIP200/220/240 is presently preferred.

While the invention is susceptible to various modifications andalternative forms, specific examples thereof have been shown by way ofexample in the drawings and are herein described in detail. It should beunderstood, however, that the invention is not to be limited to theparticular forms or methods disclosed, but to the contrary, theinvention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the appended claims.

What is claimed is:
 1. A laser comprising:a totally reflective mirrorand a partially reflective mirror disposed along an optical axis; anamplification medium disposed along said optical axis between saidmirrors; a flashlamp disposed adjacent said amplification medium; aflashlamp power source electrically coupled to said flashlamp; amodulator having a variably controllable modulation gating frequencydisposed along said optical axis between said amplification medium andone of said mirrors; a control circuit electrically coupled to saidflashlamp power source and to said modulator for triggering one or morepump pulse discharges by said flashlamp and for controlling saidmodulation gating frequency of said modulator, such that said laser, inresponse to each triggering of a pump pulse discharge by said flashlamp,generates an output burst comprising a plurality of sub-pulses havingpredetermined peak powers, said sub-pulses comprising each multi-pulsedburst having predetermined, uniform peak powers; and a non-linearcrystal disposed along said optical axis for converting saidmulti-pulsed burst outputs generated by said laser from a fundamentalfrequency to a second harmonic frequency.
 2. The laser of claim 1wherein said modulator comprises an acousto-optical Q-switch and an RFdriver, said acousto-optical Q-switch being electrically coupled to saidRF driver, and said RF driver being electrically coupled to said controlcircuit.
 3. The laser of claim 2 wherein said control circuit comprisesa microprocessor programmed to provide a gated signal having variablycontrollable gating intervals to said RF driver.
 4. The laser of claim 3wherein said amplification medium comprises an Nd:YAG rod.
 5. The laserof claim 4 wherein said fundamental frequency denotes a beam wavelengthof 1.064 μm, and said second harmonic frequency denotes a beamwavelength of 532 nm.
 6. The laser of claim 5 wherein said non-linearcrystal is a potassium-titanyl-phosphate (KTP) crystal.
 7. A lasercomprising:a totally reflective mirror and a partially reflective mirrordisposed along an optical axis; an amplification medium disposed alongsaid optical axis between said mirrors; a flashlamp disposed adjacentsaid amplification medium, a flashlamp power source electrically coupledto said flashlamp; a modulator having a variably controllable modulationgating frequency disposed along said optical axis between saidamplification medium and one of said mirrors; and a control circuitelectrically coupled to said flashlamp power source and to saidmodulator for triggering one or more pump pulse discharges by saidflashlamp and for controlling said modulation gating frequency of saidmodulator, such that said laser, in response to each triggering of apump pulse discharge by said flashlamp, generates an output burstcomprising a plurality of sub-pulses having predetermined, uniform peakpowers.
 8. The laser of claim 7 further comprising a non-linear crystaldisposed along said optical axis for converting said multi-pulsed burstoutputs generated by said laser from a fundamental frequency to a secondharmonic frequency.
 9. The laser of claim 8 wherein said fundamentalfrequency denotes a beam wavelength of 1.064 μm, and said secondharmonic frequency denotes a beam wavelength of 532 nm.
 10. A method ofgenerating a frequency doubled laser output, said method comprising thesteps of:generating a laser beam source comprising, for each pulse ofpump energy, a plurality of multi-pulsed bursts, each burst comprising aplurality of sub-pulses having predetermined peak powers, wherein eachmulti-pulsed burst comprises a plurality of sub-pulses havingpredetermined, uniform peak powers; and passing said laser beam sourcethrough a non-linear crystal.
 11. The method of claim 10 wherein saidnon-linear crystal is a potassium-titanyl-phosphate (KTP) crystal.
 12. Amethod for generating a laser output comprising a plurality ofmulti-pulsed bursts having variably controllable peak powers, saidmethod comprising the steps of:intermittently delivering pulses of pumpenergy to an amplification medium disposed along an optical axis betweena totally reflective mirror and a partially reflective mirror; allowinga beam to oscillate between said mirrors upon the delivery of each pulseof pump energy to said amplification medium; and modulating theoscillation of said beam between said mirrors at a controllablemodulation gating frequency such that a laser output comprising amulti-pulsed burst results from the delivery of each pulse of energy tosaid amplification medium and each multi-pulsed burst comprises aplurality of sub-pulses having predetermined peak powers; wherein saidstep of modulating the oscillation of said beam is performed by anacousto-optical Q-switch driven by an RF driver under microprocessorcontrol; and wherein said step of modulating the oscillation of saidbeam includes the step of monitoring a fluorescence of saidamplification medium and gating said modulation when said fluorescencereaches a selected level.
 13. The method of claim 12 wherein saidamplification medium comprises a Nd:YAG rod, and wherein a flashlampdelivers said pulses of energy to said Nd:YAG rod.
 14. The method ofclaim 12 further including the step of converting said beam to a secondharmonic by allowing said beam to pass through a non-linear crystal. 15.A method for generating a laser output comprising a plurality ofmulti-pulsed bursts having variably controllable peak powers, saidmethod comprising the steps of:intermittently delivering pulses of pumpenergy to an amplification medium disposed along an optical axis betweena totally reflective mirror and a partially reflective mirror; allowinga beam to oscillate between said mirrors upon the delivery of each pulseof pump energy to said amplification medium; modulating the oscillationof said beam between said mirrors at a controllable modulation gatingfrequency such that a laser output comprising a multi-pulsed burstresults from the delivery of each pulse of energy to said amplificationmedium, and each multi-pulsed burst comprises a plurality of sub-pulseshaving predetermined uniform peak and converting said beam to a secondharmonic by allowing said beam to pass through a non-linear crystal. 16.A laser comprising:a totally reflective mirror and a partiallyreflective mirror disposed along an optical axis; an amplificationmedium disposed along said optical axis between said mirrors; aflashlamp disposed adjacent said amplification medium, a flashlamp powersource electrically coupled to said flashlamp; a modulator having avariably controllable modulation gating frequency disposed along saidoptical axis between said amplification medium and one of said mirrors;a control circuit electrically coupled to said flashlamp power sourceand to said modulator for triggering one or more pump pulse dischargesby said flashlamp and for controlling said modulation gating of saidmodulator; and a fluorescence feedback circuit electrically coupled tosaid control circuit and providing a signal to said control circuit whena fluorescence of said amplification medium reaches a predeterminedlevel, such that said laser, in response to each triggering of a pumppulse discharge by said flashlamp, generates an output burst comprisinga plurality of sub-pulses having predetermined, uniform peak powers. 17.The laser of claim 16 wherein said fluorescence feedback circuitcomprises a photo-electric cell, a narrow pass filter and a comparator,said photo-electrical cell being disposed adjacent said amplificationmedium and electrically coupled to said comparator, said narrow passfilter being fixed to said photo-electric cell, and said comparatorbeing electrically coupled to said control circuit.
 18. The laser ofclaim 17 further comprising a non-linear crystal disposed along saidoptical axis for converting said multi-pulsed burst outputs generated bysaid laser from a fundamental frequency to a second harmonic frequency.19. A laser comprising:a totally reflective mirror and a partiallyreflective mirror disposed along an optical axis; an amplificationmedium disposed along said optical axis between said mirrors; aflashlamp disposed adjacent said amplification medium, a flashlamp powersource electrically coupled to said flashlamp; a modulator having avariably controllable modulation gating frequency disposed along saidoptical axis between said amplification medium and one of said mirrors;a control circuit electrically coupled to said flashlamp power sourceand to said modulator for triggering one or more pump pulse dischargesby said flashlamp and for enabling said modulation of said modulator;and a fluorescence feedback circuit electrically coupled to saidmodulator and providing a signal to said modulator when a fluorescenceof said amplification medium reaches a selected level, such that saidlaser, in response to each triggering of a pump pulse discharge by saidflashlamp, generates an output burst comprising a plurality ofsub-pulses having predetermined, uniform peak powers.
 20. The laser ofclaim 19 wherein said fluorescence feedback circuit comprises aphoto-electric cell, a narrow pass filter and a comparator, saidphoto-electrical cell being disposed adjacent said amplification mediumand electrically coupled to said comparator, said narrow pass filterbeing fixed to said photo-electric cell, and said comparator beingelectrically coupled to said modulator.
 21. A control circuit for alaser, said control circuit comprising:a microprocessor and afluorescence feedback circuit; said fluorescence feed back circuitproviding to said microprocessor a signal indicative of a gain of saidlaser; and said microprocessor being programmed to provide a modulationsignal having a controllable gating interval to a modulator of saidlaser, said controllable gating interval being determined in response toa state of said signal indicative of said gain of said laser.
 22. Thecontrol circuit of claim 21, wherein said fluorescence feedback circuitcomprises a photo-electric cell and a comparator, said photo-electriccell being electrically coupled to said comparator and said comparatorbeing electrically coupled to said microprocessor.
 23. The controlcircuit of claim 22 wherein a narrow pass filter is fixed to saidphoto-electric cell.
 24. The control circuit of claim 22, wherein saidmicroprocessor provides a reference signal to said comparator, and saidreference signal determines a reference voltage of said comparator. 25.A control circuit for a laser, said control circuit comprising:amicroprocessor and a fluorescence feedback circuit; said microprocessorbeing electrically coupled to a modulator of said laser and beingprogrammed to provide a modulation enabling signal to said modulator;and said fluorescence feedback circuit being electrically coupled tosaid modulator of said laser and providing a modulation gating signal tosaid modulator, said modulation gating signal being indicative of a gainof said laser and controlling a gating of said modulator.
 26. Thecontrol circuit of claim 25, wherein said fluorescence feedback circuitcomprises a photo-electric cell and a comparator, said photo-electriccell being electrically coupled to said comparator and said comparatorbeing electrically coupled to said modulator.
 27. The control circuit ofclaim 26 wherein a narrow pass filter is fixed to said photo-electriccell.
 28. The control circuit of claim 26, wherein said microprocessoris electrically coupled to said comparator and provides a referencesignal to said comparator, said reference signal determining a referencevoltage of said comparator.